This invention relates generally to power generation systems using a Rankine cycle. More particularly this invention relates to power generation systems using a Rankine cycle with a mixture of at least two liquids as the working fluid.
Rankine Cycles use a working fluid in a closed cycle to gather heat from a heating source or a hot reservoir by generating a hot gaseous stream that expands through a turbine to generate power. The expanded stream is condensed in a condenser by rejecting the heat to a cold reservoir. The working fluid in a Rankine cycle follows a closed loop and is re-used constantly. The efficiency of Rankine Cycles such as Organic Rankine Cycles (ORCs) in a low-temperature heat recovery application is very sensitive to the temperatures of the hot and cold reservoirs between which they operate. In many cases, these temperatures change significantly during the lifetime of the plant. Geothermal plants, for example, may be designed for a particular temperature of geothermal heating fluid from the earth, but lose efficiency as the ground fluid cools over time, thereby shifting the plant operating temperature away from its design point. Air-cooled ORC plants that use an exhaust at a constant-temperature from a larger plant as their heating fluid will still deviate from their design operating conditions as the outside air temperature changes with the seasons or even between morning and evening.
Therefore there is a need for a power generation system using a Rankine Cycle that can deal with fluctuations in the temperature of the hot and cold reservoir or heat sources without adversely affecting the efficiency or the stability of the power generation system.